The present invention relates to fuel systems for gas turbine engines and, more particularly, to pumps for delivering fuel for combustion. Turbine engines operate by passing a volume of gas through a series of compressors and turbines in order to produce rotational shaft power. The shaft power is used to turn a turbine for driving a compressor to provide air to a combustion process that generates high energy gases. Additionally, the shaft power is used to power a secondary turbine to, for example, drive a generator for producing electricity, or to produce high momentum gases for producing thrust. The compressed air is mixed with a fuel and combusted within the engine to generate high energy gases needed to sustain rotation of the turbines. Initial rotation of the turbines is provided by a starter that operates until the combustion process is initiated. Fuel is delivered to the engine by a fuel system that includes a fuel pump and a fuel metering system.
Typical fuel pumps comprise high pressure, fixed volume gear pumps that require mechanical rotation of the engine to produce fuel flow. As such, during a start up operation of the gas turbine engine, gear pumps are operated through the starter at speeds much lower than operating speeds of the engine. In order to deliver the required volume of fuel to the engine to initiate operation, the gear pump must be sized to have a large displacement. However, once the engine is started and rotating at operating speeds, the displacement of the gear pump provides an excess volume of fuel flow. Part of the fuel flow is dispensed to the engine through the fuel metering system, which reacts to engine needs. The fuel metering system returns excess fuel flow to the fuel system upstream of the fuel pump. The gear pump and the fuel metering system, however, produce inefficiencies in the operation of the gas turbine engine. For example, the gear pump has excess capacity for the needs of most operations of the engine, adding unnecessary size and weight. The fuel metering system adds unnecessary heat to the fuel, which can lead to gumming and coking of the engine.
Various systems have been developed to improve fuel delivery for slow shaft speeds such that mechanical fuel pumps can be reduced in size. For example, hybrid systems use mechanical and electrical pumps to produce multiple fuel flows at different engine speeds. However, such systems still require a fuel metering system to deliver the fuel to the engine at low speeds. Improvements to such systems are needed to reduce overall fuel system size, weight and efficiency.